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Hi,
Here's a post I did on another Forum which may help explain why the Oil gets so dirty. Hope some find it valuable...
Hi,
A couple of misconceptions here. Your Oil performs 3 Functions in your Car; it provides Lubrication - both Topically and Hydrostatically, it removes Dirt from the Engine, and it provides clearly 60% of the Engine's Cooling (your Reciprocating Gear - Crankshaft, Mains, Big-Ends, Wristpins all rely on the Oil to carry away the Heat - these are not Water-cooled).
So far as Lubrication, your Oil actually performs better at this as it ages - up to 15k mi. This has been studied and proven by the SAE (Society for Automotive Engineers), the Ford Motor Company and Conoco-Phillips Oil Company see: SAE Technical Paper 2003-01-3119. This testing consisted of sampling the Oil in an Engine at various mileage intervals and testing for Metal Content (an indication of wear). The metal content consistently peaked in samples drawn between 1,000 (the initial sample) and 3,500 miles, and then tapered off as the mileage rose to about 10K mi. indicating that the Oil provided less lubrication in these earlier mileages. This means that chronic 3000 mi. Oil Changers are actually causing more wear to their Engines.
As counter-intuitive as this seems, it is the additives (Anti-Oxidants and Detergents) which impede the Oil's ability to Lubricate. Once these begin to break down (after about 3000 mi.) , the Oil is better able to lubricate the Engine. Because Oil performs 3 functions, it's formula is a compromise, in order to maximize it's overall performance.
So far as Cleaning, this is dependent on a number of seemingly unrelated Factors such as the type of Air Filter you use and how dusty an environment you operate in. Your Air Filter is capable of filtering particles down to a 30 micron size, yet the everyday Dust your engine encounters consists of particles ranging from about 2.5 microns to 80 microns.
In order to provide for consistent testing, the PTI (Petroleum Testing Institute) sampled Dust from a variety of areas and came up with a Standard. This Standard has subsequently been adopted by both the SAE and the ISO (International Standards Organization). One gram of this Test Dust consists of particles of the following sizes:
0 - 2.5 Microns - 5.3%
2.5 - 5 microns - 11.5%
5 - 10 microns - 11.6%
10 - 20 microns - 14.9%
20 - 40 microns - 22.4%
40 - 80 microns - 23.4%
80+ microns - 10.9%
Why is this important? Well, if the Air Filter can only trap particles larger than 30 microns, approx. 50% of the particles in the Dust your Engine encounters will pass through the Filter and get into the Engine. It is necessary to approximate this number because as an Air Filter ages and becomes more restricted (meaning the holes in the media become smaller), it actually filters smaller particles (but restricts the amount of Air it can actually Flow).
So, for every 1.0 grams of dust your engine encounters, .5 grams gets into the Engine. The Oil will trap much of this in suspension and changing your Oil removes this Dirt along with it (BTW, much of this Dust is made up of silicates (which are fairly Hard) and consequently are very abrasive to the Engine Internals). The need to remove this Dirt has much more to do with the Oil's Service Life than any breakdown in it's lubricating qualities. Also, further testing revealed that changing the Filter every 5k mi., with it's accompanying Top-Up of Oil extends the Oil's lubricating capacity well beyond 15k mi.
Also, realize the Oil Change Interval described in your Owners Manual denotes operating under Ideal Conditions. These Ideal Conditions include non-dusty areas, driving no less than 10 miles every time you start the Engine, no or minimal Stop & Go traffic. And this is for every mile the Oil is in Service. Consequently, the only place these Ideal Conditions are ever actually met are in the laboratory and so you need to shorten your Change Interval from what the Manual dictates.
The results of this Testing indicate that the best change interval is between 7,500 mi. and 10k mi. or annually if under that mileage. Changing the Oil within this interval will result in the maximum performance of your Oil and minimum wear to your Engine. Hope this helps...
Happy Motoring!... Jim'99
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